1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular to a threshold value setting process in measuring a measurement image.
2. Description of the Related Art
An electrophotographic image forming apparatus has image forming units for respective color components which form images using toners of different colors. Toner images formed by the image forming units for the respective color components are transferred in a superposed manner to an image carrier. The image forming apparatus transfers the toner images superposed on top of one another on the image carrier to a recording material and then fixes the toner images to the recording material through heat and pressure of a fixing device to generate full-color printed matter.
In such an image forming apparatus, if images of the respective colors are out of register when they are transferred in a manner being superposed on top of one another onto an image carrier, color misregistration of the images on a recording material will occur. To address this problem, those which form a measurement image on an image carrier, and based on a result obtained by detecting this measurement image, adjust positions and timings of images to be formed for each image forming unit are known.
For example, when there is not much difference between an intensity of reflected light from an image carrier and an intensity of reflected light from a measurement image formed using a toner of a predetermined color, it is difficult for an optical sensor to detect the measurement image. For this reason, an image forming apparatus forms a composite pattern in which a measurement image of a predetermined color is superposed on a measurement image of another color (other than the predetermined color) and transferred and corrects for a color misregistration based on a detection result of the composite image by an optical sensor (Japanese Laid-Open Patent Publication (Kokai) No. 2012-3234). It should be noted that a reflectivity of the other color is higher than that of the predetermined color.
The image forming apparatus described in Japanese Laid-Open Patent Publication (Kokai) No. 2012-3234 obtains sensor output values output from an optical sensor during movement of an image carrier. The sensor output values of the optical sensor includes a sensor output value corresponding to reflected light from the image carrier and sensor output values corresponding to reflected light from the composite pattern. Further, the sensor output values corresponding to the reflected light from the composite pattern includes a sensor output value corresponding to reflected light from the measurement image of the predetermined color, a sensor output value corresponding to reflected light from the measurement image of the other color, and a sensor output value corresponding to reflected light from an area in which the measurement image of the predetermined color overlaps the measurement image of the other color. The sensor output values of the optical sensor are converted into binary signals based on a threshold value, and a color misregistration is corrected based on the signals.
FIG. 7A is a view showing the relationship between sensor output for a composite pattern and sensor output binary-coded using a threshold value. FIG. 7B is a view showing the composite pattern.
When the density of a measurement image of the predetermined color (for example, a black (K) measurement image) decreases, a sensor output value corresponding to reflected light from the composite pattern may exceed a threshold value as shown in FIG. 7A. This is because when the density of the black image decreases in an area where a magenta (M) image and the black image overlap, the magenta image is seen through the black image. As a result, the sensor can receive reflected light from the magenta image covered with the black image, thereby causing sensor output corresponding to the area where the magenta image and the black image overlap to be higher than the threshold value.
For this reason, when the density of the measurement image of the predetermined color decreases in the composite pattern, the sensor output corresponding to an area of the other color covered with the toner of the predetermined color in the composite image exceeds the threshold value, and hence it is difficult to correct for a color misregistration with high accuracy.